This application relates to a method of forming a turbine blade with triangular/trapezoidal serpentine cooling passages with a unique tooling die construction.
Turbine blades are utilized in gas turbine engines. As known, a turbine blade typically includes a platform, with an airfoil shape extending above the platform to the tip. The airfoil is curved, extending from a leading edge to a trailing edge, and between a pressure wall and a suction wall.
Cooling circuits are formed within the airfoil body to circulate cooling fluid, typically air. One type of cooling circuit is a serpentine channel. In a serpentine channel, air flows serially through a plurality of paths, and in opposed directions. Thus, air may initially flow in a first path from a platform of a turbine blade outwardly through the airfoil and reach a position adjacent an end of the airfoil. The flow is then returned in a second path, back in an opposed direction toward the platform. Typically, the flow is again reversed back away from the platform in a third path.
The location and shape of the paths in a serpentine channel has been the subject of much design consideration.
During operation of the gas turbine engine, the cooling air flowing inside the paths is subjected to a rotational force. The interaction of the flow through the paths and this rotational force results in what is known as a Coriolis force which creates internal flow circulation in the paths. Basically, the Coriolis force is proportional to the vector cross product of the velocity vector of the coolant flowing through the passage and the angular velocity vector of the rotating blade. Thus, the Coriolis effect is opposite in adjacent ones of the serpentine channel paths, dependent on whether the air flows away from, or towards, the platform.
To best utilize the currents created by the Coriolis effect, designers of airfoils have determined that the flow channels, and in particular the paths that are part of the serpentine flow path, should have a triangular/trapezoidal shape. Essentially, the Coriolis effect results in there being a primary flow direction within each of the flow channels, and then a return flow on each side of this primary flow. Since the cooling air is flowing in a particular direction, designers in the airfoil art have recognized the heat transfer of a side that will be impacted by this primary direction will be greater than on the opposed side. Thus, trapezoidal shapes have been designed to ensure that a larger side of the cooling channel will be impacted by the primary flow direction.
To form cooling channels, a so-called lost wax molding process is used. Essentially, a ceramic core is initially formed in a tooling die. Wax is placed around that core to form the external contour of the turbine blade. An outer mold, or shell is built up around the wax using a ceramic slurry. The wax is then melted, leaving a space into which liquid metal is injected. The metal is then allowed to solidify and the outer shell is removed. The ceramic core is captured within the metal, forming the blade. A chemical leeching process is utilized to remove the ceramic core, leaving hollows within the metal blade. In this way, the cooling passages in the blade are formed.
There are challenges in forming triangular/trapezoidal cooling channels using existing methods. As shown in FIG. 1A, a standard blade 20 may have a number of cooling passages. One set of cooling paths 22, 24, 26, 28 and 29 is a serpentine cooling circuit. As can be appreciated as for example in FIG. 1B, air flows outwardly and back inwardly within the blade through the serpentine circuit. As shown in FIG. 1A, ribs 31 separate the paths 22, 24, 26, 28 and 29. In the FIG. 1A embodiment, the ribs 31 are all generally parallel to each other. Other ribs 33 are non-parallel to the ribs 31, and include additional cooling passages at both a leading edge 35 and a trailing edge 37. A pressure wall 32 of the blade will face a higher pressure fluid flow when the blade is utilized in a turbine, and a suction wall 130 will face a lower pressure flow.
As mentioned, due to the Coriolis effect, as the blade rotates, the heat transfer characteristics will differ dependent on whether the air is moving outwardly or inwardly relative to the platform.
Thus, as shown in FIG. 2, it has become desirable to form a turbine blade 40 such that the paths 122, 124, 126, 128 and 130 are no longer formed between generally parallel ribs. Instead, the ribs 42 and 142 are generally at non-parallel angles relative to each other and such that the passages are triangular/trapezoidal in section. Similarly, ribs 44 adjacent the trailing edge may also be non-parallel to the ribs 42 and parallel to rib 142.
As shown schematically in FIG. 3, and as mentioned above, to form the turbine blade, a ceramic core C is initially formed in a process that will be described below. The ceramic core C is then placed into a lost wax mold, and the blade is formed as described above.
The prior art core to make the blade of FIG. 1A is formed by a process shown in FIGS. 4A-4C. As shown, a first die half 50 and a second die half 52 are brought together to define internal passages that receive ceramic material. As shown, the first die half 50 has rib extensions 54 and the second die half 52 has rib extensions 56. Together, the rib extensions 54 and 56 will form a space for ribs 31. Inserts 58 and 59 form the ribs 33 at the leading edge, and inserts 60 and 61 will form the ribs 33 at the trailing edge.
As shown in FIG. 4B, the two die halves 50 and 52 are initially brought together. As can be appreciated, the rib extensions 54 and 56 abut. Spaces 70 will form the portion of the ceramic core that will eventually form the paths in the turbine blade.
As shown in FIG. 4C, the inserts 58 and 59 and 60 and 61 are now brought together. Their extensions 69 also abut. Ceramic may now be injected into the die, and the ceramic core, such as shown in FIG. 3 will then be formed. As seen in FIG. 3, a tie bar T and upper tie bar T connect the spaces 70, although they are not shown in FIGS. 4A-4C.
At the end of formation, the process proceeds in the reverse direction with the inserts 58-59 and 60-61 being moved away from each other, and the die halves 50 and 52 then being moved away from each other, leaving the ceramic core. As can be appreciated, it would be impossible to withdraw the extensions 54 and 56 if they were at an angle that was non-parallel to a direction of movement of the die halves. As such, this prior art molding process cannot be utilized to make the FIG. 2 passages with the non-parallel ribs.